The invention relates to high frequency networks, more particularly to variable oscillators, operating at relatively high frequencies, such as colpitts oscillators. Such oscillators, or similar networks, include crystals in the frequency controlling circuits, tank circuits in the case of an oscillator, and a variable impedance in this circuit for determining the warp range, or operating range, of the oscillator. Voltage variable capacitors are ordinarily used in these circuits so that the frequency of the oscillator can be controlled by a remotely generated voltage. The range of voltage variable capacitors is limited and, until relatively recently, it has been difficult to obtain the desired warp range for the oscillator when using the voltage variable capacitor.
When the relatively wide warp range was obtained in crystal oscillators, difficulties have arisen because of the crystal having a tendency to be nonlinear when utilized over such a frequency range. The nonlinearly was caused by the physical capacitance C.sub.o, of the crystal and was improved by neutralizing the C.sub.o as by the appropriate connection thereto of an inductance coil. The tendency of the oscillator to oscillate at spurious modes, that is to be unstable, was largely overcome by the use of additional inductance coils appropriately connected.
These problems of the prior art have largely been solved by the invention disclosed in the U.S. Pat. No. 4,001,724, Anthony F. Keller, entitled "Variable High Frequency Crystal Oscillator," issued Jan. 4, 1977, and assigned to the same assignee as the present invention. In the Keller patent a pair of inductance coils are provided which are connected together and across the crystal in the circuit for neutralizing the C.sub.o of the crystal and by connecting the juncture between the two coils to one terminal of the remainder of the tank circuit for impedance matching purposes.
Knowing the impedance of the crystal and the impedance of the remaining network, tank circuit, the principles of the Keller U.S. Pat. No. 4,001,724 enabled the selection of two inductances to achieve the indicated purposes.
In variable high frequency crystal oscillators, such as disclosed in Keller, the frequency variation, or deviation range, of the oscillator, has been determined by varying the capacitance of a voltage variable capacitor, or varactor. While a high degree of accuracy is maintainable in the manufacture of crystals, varactors, and inductance coils, there is nevertheless a permissible tolerance in the manufacture of these components as well as of others. When the crystal, the varactor and the inductance coils, etc. are assembled together it is necessary, of course, that the resulting frequency deviation characteristic relative to the reactance variation producing it be within prescribed specifications. This is done by the appropriate selections of the two inductance coils referred to. If when assembled, the resulting circuit has the desired sensitivity, that is the proper relationship between reactance variations and frequency deviation, the resulting circuit is within specifications and is correct. But if not, the only alternative is to make a different selection of the two inductance coils and this presents a time consuming expensive, and largely unacceptable, procedure.